A bearing unit is generally designed to provide a smooth rotation under fluid lubricated condition, and a minimum oil film thickness or a minimum clearance between a rotating shaft and a sliding bearing is about 0.5 to 1 .mu.m.
It is to be noted, however, that there is a manufacturing tolerance on the order of about 1 to 5 .mu.m for the rotating shaft, a similar manufacturing error for the sliding bearing, and also there is an assembly tolerance as caused by such factors as a tilting of the rotating shaft and so on which may be on the order of about 1 to 3 .mu.m. A total error W is obtained as a squared mean of these values, which is generally in a range from 1.7 to 7.7 .mu.m.
Because the magnitude of the total error W is greater than the minimum thickness of the oil film as described above, it has been the practice in the prior art to provide a soft overlay layer on the surface of the sliding bearing in order to accommodate for a difference therebetween (see, for example, Japanese Laid-Open Patent Application No. 205,014/1985).
A bearing unit is also known in the art including a sliding bearing including a bearing alloy layer having a plurality of axially spaced, circumferentially extending annular grooves formed around its inner peripheral surface to define circumferentially extending peaks between pairs of axially adjacent annular grooves, and a rotating shaft which is rotatably journalled by the sliding bearing (refer Japanese Patent Publication No. 11,530/1988).
As mentioned, the provision of a soft overlay layer is required in order to accommodate for a total error W, and if such overlay layer is not provided, a seizure resistance is degraded as a result of localized abutments.
In the bearing unit described in the cited Patent Publication in which annular grooves are formed around the inner peripheral surface of the sliding bearing, each of these annular grooves is utilized as a path for a lubricating oil, and the peak defined between a pair of axially adjacent annular grooves is not utilized as means to accommodate for the total error W.